1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a storage device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, or a light-emitting device each including an oxide semiconductor.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
In recent years, with the increase in the amount of data manipulated, a semiconductor storage device having high storage capacity (hereinafter referred to as a storage device) has been required. A storage device is roughly classified into a volatile storage device that loses stored data when power is not supplied and a non-volatile storage device that retains stored data even when power is not supplied.
A typical example of a volatile storage device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a storage element is selected and charge is accumulated in a capacitor. When data is read from a DRAM, charge in a capacitor is lost according to the principle; thus, another writing operation is necessary every time data is read. A transistor included in a storage element has leakage current (off-state current) or the like between a source and a drain in an off state and charge flows into or out even when the transistor is not selected, so that a data retention period is short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption.
A typical example of a nonvolatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region of a transistor and stores data by holding charge in the floating gate. Thus, a flash memory has advantages in that a data retention period is extremely long (semi-permanent) and refresh operation which is necessary in a volatile storage device is not needed. However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, so that the storage element stops its function after a predetermined number of times of writing.
When a transistor including an oxide semiconductor in a channel formation region (hereinafter referred to as an oxide semiconductor transistor) is used as a switching element of a memory cell of a DRAM, the oxide semiconductor transistor has extremely low leakage current in an off state (off-state current); thus, a low-power DRAM having a low refresh frequency can be formed (Patent Document 1).
Patent Document 2 discloses a nonvolatile storage device including an oxide semiconductor transistor. Unlike the flash memory, the storage device has unlimited cycling capability, can easily operate at high speed because data is written by turning on or off the transistor, and consumes less power.
Patent Document 2 also discloses the structure of a storage device that stores multilevel data by using an oxide semiconductor transistor. In general, the allowable power fluctuation range of data in a multilevel memory is smaller than that of a binary memory, and small deterioration of data leads to memory malfunction. Thus, the secured data retention period of the multilevel memory is shorter than that of the binary memory.